In various fields of technology, powdery materials have to be dried and/or burned (calcined) in order to remove surface humidity and/or chemically bound water, and in order to prepare material for further processing.
For example, in the gypsum industry, it is necessary to dry the natural gypsum, which was excavated above or below ground, and which has a quarry or inherent water content of about one to three per cent by weight, in order first to remove the adsorbed surface humidity and in order to subsequently calcine, with different calcium sulfate hydrate qualities being formed as a function of the temperature and time.
Numerous procedures and devices are known for drying and calcining natural gypsum.
In contrast, problems arise when treating particularly fine powdery materials, especially calcium sulfate products, during the treatment of synthetic calcium sulfate materials, such as the so-called chemical and desulfurization gypsums. The latter are the end product obtained during desulfurization, for example during a washing procedure with limestone (CaCO.sub.3) or with calcium hydroxide (Ca(OH).sub.2) or with calcium oxide (CaO). Such desulfurization gypsum is called calcium sulfate dihydrate (CaSO.sub.4. 2H.sub.2 O).
Such desulfurization gypsum differs in many ways from natural gypsum. Already in the raw or crude state, the desulfurization gypsum is much more finely crystallized and has due to the method of production, grain sizes of sometimes much less than 50 .mu.m. The crystalline appearance also differs from that of natural calcium sulfate dihydrate. Furthermore, the adsorbed surface humidity of desulfurization gypsum is approximately ten per cent by weight greater than that of natural gypsum (about one to three per cent by weight).
Obviously, such products cannot be treated by conventional drying and/or calcining procedures and devices. Even the transport of the finely grained and moist desulfurization gypsum causes difficulties due to the undesirable "formation of lumps", which quickly results in the clogging of conveying units and transport mechanisms. The moist desulfurization gypsums are therefore dried prior to delivery to customers (discharge systems), because available conveying and calcining units cannot operate in a disturbance-free manner with moist material (Zement Kalk Gips 5 (1983) 271, 273).
However, the drying itself causes difficulties due to the described properties of the synthetic calcium sulfate products, and relatively expensive dryers, such as centrifugal hydroextractors, are used to aid in the process, though they permit only a partial removal of water. Thereafter, transport is in fact somewhat easier, but the described technical difficulties have not yet been eliminated. Moreover, the material, which is still very finely grained, cannot be calcined in conventional calcining equipment without further preparation.
Projects known up to now (Zement Kalk Gips (see above); Umwelt 6 (1983), 435) propose a nodulizing or agglomeration and a subsequent comminution or pulverizing to improve the unfavorable grain structure and the particle structure, as well as to influence the rheological characteristics. The agglomeration is to be carried out with briquetting presses or with pelletizing machines, which lead to coarsening of the grain.
Thus, the material, which is still moist, has to be additionally treated. Supplementary equipment is necessary. Such equipment is not only expensive, but considerably increases the cost of preparation of the aforementioned products, and in addition is susceptible to breakdown.
Another proposal (Umwelt, see above) proposes the production of material in pieces or lumps by pan mills: however this results in the same drawbacks.
The known procedures for preparing such finely grained and powdery materials have, for the aforementioned range of application, have the further drawback that already for reasons of cost, the compacting mechanisms required for producing a material that can be transported to cement or gypsum plants cannot be coupled with each desulfurization plant.
Other solutions have not yet become known, although there is an urgent need in this regard. Pursuant to recently passed regulations for large-furnace plants, as part of the Federal Air Pollution Control Law, power stations have to be equipped with desulfurization units by Jan. 1, 1988, and old plants have to be appropriately retrofitted, as a result of which a considerable amount of the aforementioned desulfurization gypsum will be produced.
DE-OS No. 31 29 878 discloses a procedure for producing gypsum from the desulfurization mud of desulfurization plants, with the desulfurization mud being calcined in layers as cakes. Also during this procedure the finely grained starting material will receive a new form, here layer cakes, prior to the calcining. The laYer cakes present a greater volume than do pellets or briquets. As a result, a uniform and efficient calcining is made more difficult.
It is an object of the present invention to provide a procedure and a device for drying and/or calcining powdery material that enable, at a lower expense for apparatus and energy, an easier production especially of finely grained and powdery material having a grain fraction that is predominantly below 100 .mu.m. In this connection, the possibility should preferably also be provided for being able to work continually and/or in an energy-saving manner.
In contrast to the previously disclosed proposals, the present invention recognizes that it is not necessary to solve the aforementioned problems that occur during the drying and/or calcining of powdery, especially fine-grained, material (pursuant to the present invention this means powdery material having a particle fraction that predominantly has a grain size of less than 100 .mu.m, especially powder where a predominant fraction of the particles has a grain size of less than 50 .mu.m) by first predrying the powdery material in special drying units and then compacting them for the main drying or calcining. Rather, the present invention foresees direct feeding of appropriately treated material to a drying and/or calcining unit. This applies in particular for powdery materials of the aforementioned type that have a large content of adsorbed water, such as synthetic calcium sulfate hydrate products obtained during desulfurization in power plants.
It was completely surprising that such materials could be directly introduced into a heated treatment area, without any prior supplementary preparation such as predrying and/or agglomerating, if this treatment area is divided by an essentially horizontal base, and if the base is embodied as an air/gas-permeable vibration or oscillation table.
The feeding of the aforementioned powdery, finely grained materials can be carried out in that state in which the material exists at the operating site; thus, for example, in the case of so-called desulfurization gypsums, in the form of a more or less humid mud. It has been shown that in the beginning a forced dispersal or loosening of the possibly agglomerated, very fine particles can be achieved by guiding hot gases (hot air) through the appropriately permeable base (which hence operates as a heat transfer surface) if, at the same time, the base through which the gas flows is embodied as a vibration or oscillation table, in other words, if the base executes a continuous oscillation or vibration movement at as low an amplitude as possible. Then, with an appropriate adjustment of the oscillation amplitude, the size, shape, and number of the openings in the base, and/or the speed and temperature of the hot gas that passes through, the material that is to be treated receives a greater or lesser "fluidization", so that it is transported over the base. The transport of the material from a feed mechanism to a generally oppositely disposed withdrawal mechanism is thereby effected by an appropriately oriented discharge direction of the hot gases from the intermediate base and/or the direction of movement of the vibration or oscillation table.
Correspondingly, advantageous embodiments of the present invention provide a base that is movable in pulsation in an involute oscillation, with the longer involute amplitude or side indicating the direction of transport of the material.
In the same way, it also possible to influence the direction and speed of transport by an appropriate configuration of the hot gas discharge openings of the base. If the intermediate base is made of sheet metal, preferably sheet steel, a precise stream direction can be easily achieved by forming the openings as sections that are stamped in the direction toward the upper portion of the treatment area and that are opened only in the direction of transport, with the upwardly stamped or pressed portions of the sheet metal serving to guide or deflect the hot air/hot gas. Thus, the production of the intermediate base is not effected by a simple stamping, but rather by a pressing out, similar to the situation of a rasp. The openings are preferably very small and are disposed very close to one another in order to enhance the fluidization effect of the material that is guided over the openings.
Similarly, pores and openings of sintered materials, which are also proposed as base material pursuant to the present invention, can be produced with a precise orientation. The same is true for porous ceramic materials, which are especially proposed for temperatures over 600.degree. C.
Pursuant to a further advantageous embodiment of the present invention, the rake or angle of inclination of the base can be changed, in other words, for example, the base can also rise in the direction of transport The angle of inclination is preferably infinitely variable, so that especially in combination with the stream speed of the hot gases, it is also possible to have a regulation/control of the transport speed of the material that is guided over the base.
In order to move the intermediate base, two alterative embodiments of the present invention provide for appropriate oscillation and vibration units that are disposed either directly on the base, or the forced agitator is mounted at the treatment area, so that the latter is subjected to the entire described vibration, which has the advantage that material that may adhere to the walls of the treatment area is again shaken therefrom. In both cases, the vibration or oscillation table and/or the treatment area are mounted with suspension or elastically.
By the possibility of using ceramic material for the base, it is also possible to use the inventive device in a high temperature range of over 1000.degree. C. The treatment area is then preferably entirely provided with a refractory lining, for example of suitable ceramic fibrous material.
An advantageous further development of the present invention provides for arranging burners beneath the transport base for the purpose of heating an air guidance. It has been proven that the speed of the combustion gases coming from the burners is sufficient to achieve a gas guidance through the gas-permeable base in the desired manner. Suitable burners are selected as a function of the desired temperature and size of the openings in the base. Among others, high-pressure atomization burners, low-pressure torches, and also spin-stabilized burners have been shown to be suitable. The burners are preferably disposed parallel to the intermediate base in order to achieve a uniform temperature over the surface of the base. If necessary in order to enhance the heating, burners could also be disposed in the region above the intermediate base.
An alternative embodiment of the present invention provides for disposing the burners outside the treatment area, i.e. below and in front of appropriate openings in the bottom of the treatment area. This ensures that no oscillations are transmitted from the treatment area to the burners. At the same time, the air gap between the burners and the wall of the treatment area can be used for the supply of secondary air, as a result of which a type of injection effect is created. It is, of course, also possible to couple the burners and the treatment area in a gastight manner via an appropriate flexible coupling. Instead of heating with burners, it is also possible to provide an indirect heating, for example an electric heating (microwaves) or an infrared heating. In such a case, additional fans are provided in the treatment area in order to convey the air/gas through the porous base.
It is also proposed pursuant to the present invention to divide the treatment area into different zones or sections between the feed and withdrawal mechanisms and hence in the direction of transport of the material that is to be treated This is effected in one advantageous inventive embodiment by, for example, slides or plates that are movable from the top of the upper section of the treatment area in the direction toward the intermediate base, thereby on the one hand assuring that the material can flow over the base in an unobstructed manner, and on the other hand, however, appropriate sections or zones of different humidities can be formed along the transport path in conformity with a progressive drying effect, thereby on the whole increasing the effectiveness of the device. These structural measures especially exhibit their effectiveness if at least one suction mechanism, which can preferably be regulated and/or controlled, is provided in the upper portion of the treatment area, so that the more or less humid exhaust gases, which are more or less enriched with solid particles, can be withdrawn from the treatment area. In this way, the efficiency of the inventive device is further optimized, especially if each individual section within the treatment area is equipped with its own suction mechanism.
Pursuant to another structural embodiment of the present invention, the transport lines that are connected to the individual suction mechanisms are guided into one or more dust collection units. Here it is possible not only to free the exhaust gases from dust, but, pursuant to a further embodiment, the merging hot air can be supplied to a heat exchanger that is coupled with one or more of the burners, and the dust that is collected can be guided back to the feed and/or withdrawal mechanism of the device.
Especially by means of the selective return of the solid material recovered from the dust collection unit, the control of the drying and/or calcining procedure can be set as a function of the respective degree of drying and/or calcining.
Pursuant to a further advantageous specific embodiment of the present invention, the feeding and withdrawal of the material that is to be treated can be adapted to the novel drying and/or calcining device, for which purpose the appropriate transport means are to be embodied as vibration and/or air conveying chutes. In this connection, it is particularly advantageous if the appropriate transport means, similar to the previously described device, are provided with a gas-permeable transport base through which hot gases are guided from below. As a result, a certain loosening or dispersal and predrying can already take place as the material is fed into the device.
The feed and withdrawal mechanisms, as well as the other described connections to the device, are preferably coupled to the treatment area in a dusttight and/or gastight manner.
The inventive device can be used not only just as a dryer, but exclusively for calcining the materials. In conformity with the required treatment temperatures, merely the materials for the treatment area and its components as described above, have to be adapted, with the rest of the construction remaining the same.
In order to dry and/or calcine materials, the material stream can also be conveyed several times through one and the same device, in which connection at least one partial stream is conveyed back from the withdrawal mechanism to the feed mechanism.
Especially where the same material stream is conveyed several times through the inventive device, it was totally surprising that although at first a loosening of the material takes place pursuant to the inventive procedure, and hence a particularly uniform and intensive drying/calcining is effected, as the retention time of the particles in the inventive device increases, an agglomeration of the fine particles to form larger structures, and hence a sort of automatic pelletizing, is again effected. The reasons for this phenomenon cannot yet be explained. However, tests have shown that in particular the surface tension of the particles, and their affinity toward one another, especially during longer retention times in the inventive device or when conveyed through several times, increases and rises respectfully to such an extent that the aforementioned automatic agglomeration occurs.
Thus, the present invention presents various drying and/or calcining procedures and pertaining devices.
For example, it is possible to use the inventive device exclusively for drying the aforementioned materials, whereby the device for the drying of desulfurization gypsum, can be coupled directly without difficulty to a desulfurization plant due to its compactness. The drying can be carried out in such a way that, especially where the material is guided through the dryer once and/or relatively shortly, finely powdery material is discharged that differs from the supplied material essentially only by a reduced content of free water, so that transport, for example in silo or tank trucks, is possible.
The material dried in this manner can be calcined with an essentially structurally identical or other known burner or calcining units.
However, it is also possible to slow down the drying in the described manner in order to finally be able to withdraw from the dryer a pelletized product that can then also be supplied to conventional calcining units.
In any case, with the least possible expense, not only is a direct drying possible on location "at the coal face" in the power station, but the physical appearance or characteristic of the product that is to be dried can be individually adjusted. If dried material in pellet form is desired, it is possible to accomplish this without using the described expensive known pre-drying and compacting devices, which in addition are susceptible to failure.
The present invention also offers the possibility of first drying the material, and then, in one in the same device, calcining the material by increasing the temperature (via the burner) and again conveying the material through the device.
For such a procedure, another advantageous specific embodiment of the present invention provides for the linking together of at least two of the mentioned devices, with a transportation connection for the material that is to be treated being disposed between the withdrawal mechanism of one device and the feed mechanism of another device connected thereto. The powdery material is transferred from a first device into a second device, where a higher temperature is established in the treatment area. Depending upon the degree of drying and/or calcining that is desired, the temperatures and the speed of transport of the material through the treatment areas can be adjusted. The transport speed is preferably achieved by an appropriate adjustment of the gas-permeable base.
Obviously, a greater flexibility during drying or calcining can be achieved by coupling several devices, whereby, as provided in one advantageous embodiment of the present invention, for example partial streams are also withdrawn along the transport path from individual parts of the device in conformity with a defined degree of calcining.
In this way, for example during the calcining of gypsum, calcium sulfate dihydrate that is largely free of adsorbed water can be withdrawn from the discharge of the first device, and calcium sulfate dihydrate or calcium sulfate beta hemihydrate can be withdrawn at the end of a further subsequent device, whereby with appropriate equipment, the pyro procedure can be carried out at least until anhydrite II is obtained.
It is particularly advantageous if the individual devices are disposed in stages relative to one another, as a result of which the transport of the powdery material from one device to the next is facilitated. However, it is also possible to dispose the devices one above the other.
Furthermore, it is possible to provide connecting lines between the individual devices to return or introduce the output of one device into the other device.
The inventive device can be adapted individually, or in combination with several units, to the respective product that is to be dried/calcined, and in particular, among other things, by: selection of the burners, the calcining temperature, regulation of the vibration/oscillation movement of the intermediate base, structural configuration of the vibration/oscillation base, arrangement of various intermediate bases relative to one another adjustment of the openings/pores of the intermediate base, the rake or inclination angle of the intermediate base, adjustment of the amplitude of the vibration/oscillation base, guidance of the exhaust gas, removal of dust and return guidance of solid material and gas, and splitting of the treatment areas into zones in the direction of transport.
The inventive device furthermore offers the advantage that high throughput capacities can be achieved with very little energy. The vibrating motors that are used, for example, require only very little actuation power in order to transmit the desired oscillations to the base through which the gas flows. The frequency of the vibrator can preferably be regulated in order to be able to individually adjust the oscillation amplitude.
Also effected as a result of the fluidization of the fine powdery material that can be achieved with the inventive device is an optimum heat transfer to the goods that are to be treated, and hence an optimization of the drying or calcining process with regard to an energy supply that is as slight as possible, with the heat quantity necessary per kilogram of drying/calcining material being able to be further optimized by the described return of the exhaust gases and their guidance into heat exchangers in order to preheat the combustion air for the burners.
Further features of the present invention are indicated by the disclosure collectively as well as the remainder of the specification. A special alternative embodiment of the inventive device/unit provides for disposing several spaced apart air/gas-permeable bases one above the other within a single treatment area, and then guiding the material several times over one and/or several bases, with the device also being equipped with several feed and withdrawal mechanisms as well as appropriate return belts or other members. With a central heating from below, different temperature conditions in conformity to different drying/calcining levels of the material can then be set between the individual bases. Of course, additional calcining units and/or suction mechanisms can be provided between the individual bases. A particularly compact space and energy saving manner of construction can be realized with the preceding embodiment.